Performance improvement is zero.
Reliability gets cut in half.
0/2 = 0.
More seriously,
At one time, I had a Intel X25-M SSD. It worked well, but I needed more space, so I bought a second.
I really wanted a 160gb single image for the "C" drive, so I combined the two in raid-0.
It worked well, but I could not detect any improvement in my user experience.
Later, I used the 80gb drives in other PC's and replaced them with a single X25-M 160gb drive, and performance was equal, if not better.
Lastly, a single ssd at 2x the capacity will not cost as much as a single larger ssd.
You will actually get slightly better performance since there are more nand blocks that can be accessed in parallel.
More importantly, the availability of more...